The disclosure relates generally to turbo-machines such as steam and gas turbines, and more particularly, to an apparatus and method for measuring deflection between rotating turbine blade tips and their surrounding casing.
Turbomachines, such as gas and steam turbines, typically include a centrally-disposed rotor that rotates within a stator. A working fluid flows through one or more rows of circumferentially arranged rotating blades that extend radially outward from the rotor shaft. The fluid imparts energy to the shaft, which is used to drive a load such as an electric generator or compressor.
Clearance between radially outer tips of the rotating blades and stationary shrouds on an interior of the stator in, e.g., compressor and turbine sections of gas turbines strongly impacts efficiency of the gas turbine engine. The smaller the clearance between the rotor blades and the inner surface of the stator, the lower the likelihood of fluid leakage across blade tips. Fluid leakage across blade tips causes fluid to bypass a row of blades, reducing efficiency.
Insufficient clearance may also be problematic, however. Operating conditions may cause blades and other components to experience thermal expansion, which may result in variations in blade tip clearance. The specific effects of various operating conditions on blade clearance may vary depending on the type and design of a particular turbomachine. For example, tip clearances in gas turbine compressors may reach their nadir values when the turbine is shut down and cooled, whereas tip clearances in low pressure steam turbines may reach their nadir values during steady state full load operation. If insufficient tip clearance is provided when the turbomachine is assembled or re-assembled after inspection/repair, the rotating blades may hit the surrounding shroud, causing damage to the shroud on the stator interior, the blades, or both when operating under certain conditions.
During turbine assembly and re-assembly after inspection/repair, the lower stator shell is typically assembled first, then the rotor is set in place. Then the upper stator shell is assembled, including affixing the upper shell to the lower shell of the stator as shown in FIG. 1. This may typically be done by, e.g., bolting arm 222 of upper stator shell 220 to arm 242 of lower stator shell 240 together in a horizontal joint 230.
Although rotor-to-stator clearances can be measured in the lower half prior to assembling the upper half (i.e., in the “tops-off” condition, see FIG. 4), these values are not directly representative of the values in the fully assembled turbine (i.e., in “tops-on” condition, see FIG. 3) because the turbine shell is supported differently when the upper shell 220 of the stator is affixed to the lower shell 240. In the tops-on condition, support is shifted from the lower shell arm 242 to the upper shell arms 222, the weight of the upper shell 220 of the stator is added, and when the horizontal joint 230 is bolted, the overall stator 200 structure stiffens. As a result of these and other changes, the rotor-to-stator clearance is different in the tops-on and tops-off conditions, by a factor which may not be readily predictable. In the tops-on condition, in which the turbomachine is operated, clearances cannot be measured directly, since the turbomachine is fully assembled, and the rotating blades and inner surface 210 of stator 200 are not accessible.
One way the tops-on/tops-off shift has been addressed has been to use clearances between the rotating blade tips and the inner surface of the stator that are sufficiently large as to include the tops-on/tops-off deviation. For reasons discussed above, however, this is detrimental to turbomachine performance and efficiency because it is likely to result in excessive clearances and leakage of working fluid across blade stages.
Another approach has been to use factory tops-on/tops-off data in the field. However, this presents a data management problem, as factory data may be taken years before the turbomachine is disassembled in the field and must be reassembled. Differences in conditions between the factory and the field further complicate this approach.